This invention relates to a method for the surface treatment of shaped articles of polyvinyl chloride resins containing low-molecular plasticizers, which surface treatment is directed to precluding the possibility of the plasticizers migrating or diffusing toward the surface and exuding on the surface of the shaped articles with lapse of time or the possibility of the plasticizers exuding out of the shaped articles and transferring into other objects with which the shaped articles are held in intimate contact.
Polyvinyl chloride resins are one class of inexpensive plastic materials used extensively in films, artificial leathers, insulating coats for electric wires and cables, floorings, tubings and plates, fibers and sponge foams. Plasticized non-rigid type articles of polyvinyl chloride resins generally contain from 15 to 50% by weight of low-molecular plasticizers. When such products are held in intimate contact with other objects of gaseous, liquid and solid states, the plasticizers transfer into such other objects and give them undesirable effects.
For example, the insulating coats used for covering electric wires are chiefly made of polyvinyl chloride resins. Since these coats are constantly exposed to the heat issuing from the passage of electric current in the wires, any low-molecular plasticizers present in the coats are vapourized into the atmosphere and cause deteriolation of the physical properties of the resin. Because of such adverse effects, low-molecular plasticizers cannot be used in such coats and it is necessary to use polymeric plasticizers. When polyvinyl chloride resins containing low-molecular plasticizers are used for interior finish such as floorings and wall coverings, the plasticizers contained therein diffuse within and exude on the surface of the floorings and wall coverings and induce the so-called phenomenon of blocking. Consequently, the exuding plasticizers cause surface staining and bring about similar other adverse effects. Further, when a shaped article of plasticized polyvinyl chloride resins are brought into liquid substances such as, for example, oils and organic solvents, the plasticizers contained in the shaped articles are extracted into such liquid substances, leaving the products with decreased plasticizer contents. Consequently, the products have their properties deteriolated by the loss of plasticizers. When such shaped articles are brought into contact with solutions containing such fat-dissolving components as soap water, foodstuffs such as milk and edible oils and living-body components such as blood and body fluids, the plasticizers exude and transfer into such substances and seriously contaminate them. Consequently, the use of such polyvinyl chloride resins as the materials of containers for cosmetic products, packaging materials for foodstuffs and materials for medical wares must be under specific care. Also in the case of agricultural purpose polyvinyl chloride resin films containing low-molecular plasticizers, the plasticizers exude to the surface of the films which they are in storage or in use and bring about the phenomenon of blocking, the decrease in the transparency to light and other undesirable phenomena.
With a view to curbing the possible transfer into other objects of the plasticizers contained in plasticized shaped articles of polyvinyl chloride resins there have been adopted various measures designed to modify fabrication technology. There have been proposed various methods for preventing the blocking and migration of plasticizers to a contacting body from the articles. Examples are (1) a method of using instead of low-molecular plasticizers non-migrating, non-extractable polymeric plasticizers such as polyester plasticizers, (2) a method of coating the flexible shaped articles of polyvinyl chloride resins with various surface treatment agents such as acrylic resins, polyurethane resins, polyamide resins and other similar resins which are prepared in the form of film-forming agents, (3) a method of adding to the resins such substances as aliphatic amides, silicone fluids, silica filler, diatomaceous earth, kaolin and talc which are effective in the prevention of the blocking phenomenon. The above described methods can not free from their respective problems. For example, polymeric plasticizers are more expensive and yet are less effective than low-molecular plasticizers. The method of coating the shaped articles with some other resins has a disadvantage that the coating operation is not easy to perform and the coat tends to peel off upon contact with other solids during a prolonged use of the articles. Addition of special components entails a sacrifice of some other desirable properties and fails to bring about the intended effect as expected. Thus, need has long been felt for development of a method which, by means of a simple treatment, provides effective prevention of the exudation of plasticizers from plasticized shaped articles of polyvinyl chloride resins.
An object of the present invention is to provide a method which, though simple in operation as compared with the conventional methods, provides effective prevention of the exudation of low-molecular plasticizers contained in shaped articles of polyvinyl chloride resins.